1. Field of the Invention
The present invention relates to a radio terminal which transmits data using a plurality of frequency bands including frequency bands preferentially licensed to other radio terminals.
2. Related Art
With the development of an information-oriented society, communication means available in familiar scenes are becoming drastically widespread and there is also a remarkable increase in a communication capacity. In order to increase the communication capacity, frequency bands used for specific applications are often made available for different applications. For example, in current general radio communications, it is stipulated by law that specific frequency bands should be used exclusively for specific applications, but several frequency bands are already open to a plurality of applications. In the future, a very wide band will be possibly made open to the public so as to be used together with other applications exclusively for small power short-distance communications such as UWB (Ultra-wideband).
Furthermore, a study of a scheme called “cognitive radio” has been started whereby a frequency band licensed to a specific system is allowed to be made available to an unlicensed radio terminal exclusively for places and times unoccupied by the specific system (see, for example, Japanese Patent No. 3583962).
A basic concept of the cognitive radio is to scan a potential band to be used, detect its status of use and use the band if the band is unoccupied by a system licensed to the band. Considering the speed enhancement and use of increasingly wider bandwidths in radio communications in recent years, it is anticipated that the ability to scan quite a wide band and determine to use a band that can satisfy demanded performance will be required.
As described above, a radio terminal which carries out communication using cognitive radio (hereinafter, referred to as a “cognitive radio set”) detects and uses an unoccupied frequency band licensed to other systems. Therefore, in the case where a cognitive radio set uses a certain frequency band, there is a problem that when a licensed radio terminal which is an original user of the frequency band (hereinafter referred to as a “licensed terminal”) starts to use the frequency band, the cognitive radio set must immediately stop the use of the frequency band and suppress the influences on data transmission to the licensed terminal.
Furthermore, when the use of the scheduled frequency band is stopped along with the start of a communication of the licensed terminal, continuity of the communication on the cognitive radio set side is not guaranteed, which results in problems that necessary file transfers cannot be completed, QoS (Quality of Service) of an isochronal communication cannot be guaranteed and so on.